Semiconductor cleaner and method of cleaning semiconductor

ABSTRACT

A semiconductor cleaner includes (a) an inner wet bath which is filled with liquid and in which a wafer to be cleaned is soaked, (b) an outer wet bath which is filled with liquid and in which the inner wet bath is soaked, (c) a megasonic-wave irradiator located outside the inner wet bath for irradiating megasonic waves to the wafer to clean the wafer, and (d) a unit which moves the megasonic-wave irradiator towards or away from the inner wet bath.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a semiconductor wafer cleaner by meansof megasonic waves, and a method of cleaning a semiconductor wafer bymeans of megasonic waves.

[0003] 2. Description of the Related Art

[0004] Supersonic waves such as megasonic waves are used in a lot offields for cleaning an object. For instance, megasonic waves are used ina fabrication process of a semiconductor device for cleaning a siliconwafer.

[0005] An example of a conventional silicon wafer cleaner for cleaning asilicon wafer by means of megasonic waves is illustrated in FIG. 1.

[0006] The illustrated silicon wafer cleaner 50 is comprised of acleaning bath 52 filled with cleaning solution 51 such as water, anoscillation plate 53 adhered to an outer surface of a bottom of thecleaning bath 52, and an oscillator 54 which oscillates the oscillationplate 53.

[0007] A silicon wafer 55 to be cleaned is kept soaked in the cleaningsolution 51 in the cleaning bath 52. By activating the oscillator 54,the oscillation plate 53 is oscillated by the oscillator 54, resultingin that megasonic waves 56 are irradiated towards the cleaning bath 52from the oscillation plate 53. The silicon wafer 55 is cleaned by thethus irradiated megasonic waves 56.

[0008] As illustrated in FIG. 1, the megasonic waves 56 are irradiatedfrom the oscillation plate 53 in all directions. Hence, the megasonicwaves 56 includes first megasonic waves 56 a irradiated upwardly fromthe oscillation plate 53, and second megasonic waves 56 b irradiatedobliquely upwardly from the oscillation plate 53.

[0009] A part of the second megasonic waves 56 b is reflected at asidewall 52 a of the cleaning bath 52, and is directed towards inside ofthe cleaning bath 52 as reflected megasonic waves 56 c. As a result, thefirst megasonic waves 56 a and the reflected megasonic waves 56 cinterfere with each other at locations indicated with solid circles 57(). This means that the silicon wafer 55 locally receives the megasonicwaves 56 having high intensity.

[0010] As mentioned above, since the silicon wafer 55 receives themegasonic waves 56 too much at the locations 57 at which the firstmegasonic waves 56 a and the reflected megasonic waves 56 c interferewith each other, there is caused a problem that wirings formed on thesilicon wafer 55 are partially broken.

[0011] Many attempts have been made so far in order to solve theabove-mentioned problem.

[0012] As an example of the solution to the above-mentioned problem,Japanese Unexamined Patent Publication No. 11-221534 has suggested asupersonic wave cleaner. FIG. 2 illustrates the suggested supersonicwave cleaner.

[0013] The illustrated supersonic wave cleaner 60 is comprised of acleaning bath 62 filled with a cleaning solution 61, a first supersonicwave oscillator 63 fixed onto an outer surface 62 a of a bottom of thecleaning bath 62, a first variable oscillator 64 which controls afrequency of supersonic waves emitted from the first supersonic waveoscillator 63, a second supersonic wave oscillator 65 designed to bemovable upwardly and downwardly relative to the cleaning bath 62, and asecond variable oscillator 66 which controls a frequency of supersonicwaves emitted from the second supersonic wave oscillator 65.

[0014] A silicon wafer 67 to be cleaned is kept soaked in the cleaningsolution 61 with being fixed to the second supersonic wave oscillator65. The silicon wafer 67 moves upwardly and downwardly in the cleaningsolution 61 together with the second supersonic wave oscillator 65.While the silicon wafer 67 moves upwardly and downwardly in the cleaningsolution 61, the first supersonic wave oscillator 63 irradiatessupersonic waves to the silicon wafer 67, and thus, the silicon wafer 67is cleaned.

[0015] It is said in the Publication that since the silicon wafer 67 iscleaned by supersonic waves propagated from both of the first and secondsupersonic wave oscillators 63 and 65 in different manners, theabove-mentioned problem caused by applying only supersonic wavesirradiated from the first supersonic wave oscillator 63 to the siliconwafer 67 can be solved.

[0016] Japanese Unexamined Patent Publication No. 61-194727 hassuggested a supersonic wave cleaner. FIG. 3 illustrates the suggestedsupersonic wave cleaner 70.

[0017] The supersonic wave cleaner 70 includes a reflector 72 inclinedby a predetermined angle θ in a cleaning bath 71. The reflector 72 isformed at a surface thereof with slight irregularity. Supersonic wavesirradiated from a supersonic wave oscillator 73 arranged on a sidewallof the cleaning bath 71 are reflected at the reflector 72, and then,irradiated to a silicon wafer 75 being kept soaked in cleaning solution74 filling the cleaning bath 71 therewith.

[0018] In accordance with the supersonic wave cleaner 70, sincesupersonic waves are irregularly reflected by the slight irregularityformed at a surface of the reflector 72, it would be possible producesupersonic waves having no directivity, but having a uniform density.Hence, it is said in the Publication that the above-mentionedinterference problem can be solved.

[0019] However, the supersonic wave cleaner 60 illustrated in FIG. 2 andthe supersonic wave cleaner 70 illustrated in FIG. 3 are accompaniedwith the following problems.

[0020] In the supersonic wave cleaner 60 illustrated in FIG. 2, since itis necessary to attach the silicon wafer 67 to the second supersonicwave oscillator 65, a cleanness of the silicon wafer 67 might bedeteriorated in a step of attaching the silicon wafer 67 to the secondsupersonic wave oscillator 65, or by the second supersonic waveoscillator 65 making contact with the silicon wafer 67.

[0021] In addition, since steps of attaching the silicon wafer 67 to thesecond supersonic wave oscillator 65 and separating the secondsupersonic wave oscillator 65 from the silicon wafer 67 have to beadditionally carried out, the number of steps for fabricating thesupersonic wave cleaner 60 would be unavoidably increased.

[0022] Furthermore, since the supersonic wave cleaner 60 has to includetwo oscillators, specifically, the first and second supersonic waveoscillators 63 and 65, the number of parts for constructing thesupersonic wave cleaner 60 is unavoidably increased, and hence, costsfor fabricating the supersonic wave cleaner 60 is also increased.

[0023] In the supersonic wave cleaner 70 illustrated in FIG. 3, thereflector 72 arranged in the cleaning bath 71 produces a useless space76 therebelow, resulting in an unnecessary increase in a size of thecleaning bath 71 and hence the supersonic wave cleaner 70.

[0024] In addition, since the reflector 72 is inclined, supersonic wavesreflected at an upper portion of the reflector 72 and supersonic wavesreflected at a lower portion of the reflector 72 reach the silicon wafer75 in different distances. Accordingly, the supersonic waves reflectedat an upper portion of the reflector 72 would have a greater attenuationfactor than the supersonic waves reflected at a lower portion of thereflector 72. Thus, it is not always possible to produce supersonicwaves having a uniform density.

[0025] Japanese Unexamined Patent Publication No. 2001-120918 hassuggested a method of cleaning a filtering plane in a solid-liquidseparator, including the steps of producing supersonic waves by means ofa supersonic wave oscillator, applying the supersonic waves to a movableoscillator, moving the oscillator relative to the filtering plane, andapplying the supersonic waves to the filtering plane to thereby cleanthe filtering plane.

[0026] However, the above-mentioned problems remain unsolved even in thesuggested method.

SUMMARY OF THE INVENTION

[0027] In view of the above-mentioned problems in the conventionalsupersonic wave cleaner, it is an object of the present invention toprovide a semiconductor cleaner which is capable of avoiding megasonicwaves from interfering with each other to thereby prevent localconcentration of megasonic waves to a semiconductor wafer to be cleaned.

[0028] It is also an object of the present invention to provide a methodof doing the same.

[0029] In one aspect of the present invention, there is provided asemiconductor cleaner including (a) a wet bath which is filled withliquid and in which a wafer to be cleaned is soaked, (b) amegasonic-wave irradiator located outside the wet bath for irradiatingmegasonic waves to the wafer to clean the wafer, and (c) a unit whichmoves the megasonic-wave irradiator towards or away from the wet bath.

[0030] In the semiconductor cleaner in accordance with the presentinvention, the megasonic-wave irradiator is moved upwardly anddownwardly relative to the wet bath with megasonic waves beingirradiated to a wafer from the megasonic-wave irradiator. As a result,megasonic waves are deviated in phases from each other, and thus,megasonic waves interfere with each other in broader areas. Hence, itwould be possible to prevent local concentration of megasonic waves to awafer.

[0031] It is preferable that the unit moves the megasonic-waveirradiator by a distance in the range of 2 mm to 20 mm both inclusive.

[0032] It is preferable that the megasonic-wave irradiator irradiatesmegasonic waves having a frequency in the range of 750 KHz to 1 MHz bothinclusive.

[0033] The wet bath may be formed at an inner surface thereof withirregularity having a surface roughness greater than a wavelength of themegasonic waves.

[0034] It is preferable that the unit moves the megasomicwave irradiatorat a rate which is not equal to multiples of a phase velocity of themegasonic waves and multiples of a half of a phase velocity of themegasonic waves, and the unit moves the megasonic-wave irradiator by adistance which is not equal to multiples of a wavelength of themegasonic waves and multiples of a half of a wavelength of the megasonicwaves.

[0035] There is further provided a semiconductor cleaner including (a) awet bath which is filled with liquid and in which first to N-th wafersto be cleaned are soaked wherein N is an integer equal to or greaterthan two, (b) first to N-th megasonic-wave irradiators located outsidethe wet bath and associated with the first to N-th wafers, respectively,for irradiating megasonic waves to the first to N-th wafers to clean thefirst to N-th wafers, and (c) first to N-th units which move the firstto N-th megasonic-wave irradiators towards or away from the wet bath,respectively.

[0036] It is preferable that the first to N-th units move the first toN-th megasonic-wave irradiators, respectively, by a distance in therange of 2 mm to 20 mm both inclusive.

[0037] It is preferable that the first to N-th megasonic-waveirradiators irradiate megasonic waves having a frequency in the range of750 KHz to 1 MHz both inclusive.

[0038] It is preferable that each of the first to N-th units moves themegasonic-wave irradiator at a rate which is not equal to multiples of aphase velocity of the megasonic waves and multiples of a half of a phasevelocity of the megasonic waves, and each of the first to N-th unitsmoves the megasonic-wave irradiator by a distance which is not equal tomultiples of a wavelength of the megasonic waves and multiples of a halfof a wavelength of the megasonic waves.

[0039] There is still further provided a semiconductor cleaner including(a) a wet bath which is filled with liquid and in which a wafer to becleaned is soaked, and (b) a megasonic-wave irradiator located outsidethe wet bath for irradiating megasonic waves to the wafer to clean thewafer, the wet bath being formed at an inner surface thereof withirregularity having a surface roughness greater than a wavelength of themegasonic waves.

[0040] By forming irregularity having a surface roughness greater than awavelength of the megasonic waves, on an inner surface of a sidewall ofthe wet bath, the megasonic waves are irregularly reflected at the innersurface of a sidewall of the wet bath. As a result, megasonic waves aredeviated in phases from each other, and thus, megasonic waves interferewith each other in broader areas. Hence, it would be possible to preventlocal concentration of megasonic waves to a wafer.

[0041] There is yet further provided a semiconductor cleaner including(a) an inner wet bath which is filled with liquid and in which a waferto be cleaned is soaked, (b) an outer wet bath which is filled withliquid and in which the inner wet bath is soaked, (c) a megasonic-waveirradiator located outside the inner wet bath for irradiating megasonicwaves to the wafer to clean the wafer, and (d) a unit which moves themegasonic-wave irradiator towards or away from the inner wet bath.

[0042] For instance, the megasonic-wave irradiator may be comprised of(c1) a megasonic-wave oscillation plate located in the outer wet bathand below a bottom of the inner wet bath in parallel with the bottom ofthe inner wet bath, and (c2) a megasonic-wave oscillator connected atone end to the megasonic-wave oscillation plate and at the other end tothe. unit, the megasonic-wave oscillator being fit into an openingformed at a bottom of the outer wet bath for slidable movement.

[0043] It is preferable that the inner wet bath is composed of quartz.

[0044] There is further provided a semiconductor cleaner including (a)an inner wet bath which is filled with liquid and in which first to N-thwafers to be cleaned are soaked wherein N is an integer equal to orgreater than two, (b) an outer wet bath which is filled with liquid andin which the inner wet bath is soaked, (c) first to N-th megasonic-waveirradiators located outside the inner wet bath and associated with thefirst to N-th wafers, respectively, for irradiating megasonic, waves tothe first to N-th wafers to clean the first to N-th wafers, and (d)first to N-th units which move the first to N-th megasonic-waveirradiators towards or away from the inner wet bath, respectively.

[0045] There is further provided a semiconductor cleaner including (a)an inner wet bath which is filled with liquid and in which a wafer to becleaned is soaked, (b) an outer wet bath which is filled with liquid andin which the inner wet bath is soaked, and (c) a megasonic-waveirradiator located outside the inner wet bath for irradiating megasonicwaves to the wafer to clean the wafer, the inner wet bath being formedat an inner surface thereof with irregularity having a surface roughnessgreater than a wavelength of the megasonic waves.

[0046] In another aspect of the present invention, there is provided amethod of cleaning a wafer soaked in liquid filled in a wet bath,including the step of irradiating megasonic waves to the wafer with arelative distance between the wafer and a megasonic-wave irradiatorbeing varied.

[0047] It is preferable that the relative distance is varied byreciprocating the megasonic-wave irradiator relative to the wafer.

[0048] It is preferable that the relative distance is varied in therange of 2 mm to 20 mm both inclusive.

[0049] It is preferable that the megasonic waves have a frequency in therange of 750 KHz to 1 MHz both inclusive.

[0050] It is preferable that relative distance is varied by moving themegasonic-wave irradiator at a rate which is not equal to multiples of aphase velocity of the megasonic waves and multiples of a half of a phasevelocity of the megasonic waves, and by a distance which is not equal tomultiples of a wavelength of the megasonic waves and multiples of a halfof a wavelength of the megasonic waves.

[0051] The advantages obtained by the aforementioned present inventionwill be described hereinbelow.

[0052] In accordance with the present invention, it would be possible toincrease the number of locations at which megasonic waves interfere withone another, and scatter the locations in a broader area. Hence, itwould be possible to reduce an intensity of megasonic waves locallyconcentrating to a wafer due to interference of megasonic waves with oneanother. Thus, it would be possible to prevent a wafer from beingdamaged due to interference of megasonic waves with one another.

[0053] The above and other objects and advantageous features of thepresent invention will be made apparent from the following descriptionmade with reference to the accompanying drawings, in which likereference characters designate the same or similar parts throughout thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0054]FIG. 1 is a schematic view illustrating a conventionalsemiconductor cleaner.

[0055]FIG. 2 is a schematic view illustrating another conventionalsemiconductor cleaner.

[0056]FIG. 3 is a schematic view illustrating still another conventionalsemiconductor cleaner.

[0057]FIG. 4 is a schematic view of a semiconductor cleaner inaccordance with the first embodiment.

[0058]FIG. 5 illustrates irradiation of megasonic waves in thesemiconductor cleaner in accordance with the first embodiment.

[0059]FIG. 6 is a schematic view of a semiconductor cleaner inaccordance with the second embodiment.

[0060]FIG. 7 is a schematic view of a semiconductor cleaner inaccordance with the third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] Preferred embodiments in accordance with the present inventionwill be explained hereinbelow with reference to drawings.

[0062] [First Embodiment]

[0063]FIG. 4 illustrates a semiconductor cleaner 10 in accordance withthe first embodiment.

[0064] The semiconductor cleaner 10 in accordance with the firstembodiment is comprised of an inner wet bath 18 which is filled withcleaning solution 11 and in which a wafer 12 to be cleaned is soaked, anouter wet bath 15 which is filled with water 14 and which surrounds theinner wet bath 13, and a megasonic-wave irradiator 16 arranged below abottom of the inner wet bath 13 for irradiating megasonic waves.

[0065] The megasonic-wave irradiator 16 is comprised of a megasonic-waveoscillation plate 17 located in the outer wet bath 15 and below a bottomof the inner wet bath 13 in parallel with a bottom of the inner wet bath13, a megasonic-wave oscillator 18 connected at one end to themegasonic-wave oscillation plate 17 for oscillating the megasonic-waveoscillation plate 17 to produce megasonic waves, and at the other end toa later mentioned motor 19, and a motor 19 which moves themegasonic-wave oscillation plate 17 and the megasonic-wave oscillator 18upwardly and downwardly relative to the inner wet bath 13.

[0066] A wafer 12 is kept soaked in the cleaning solution 11 by means ofan appropriate holder (not illustrated).

[0067] The outer wet bath 15 may be composed of any material, whereasthe inner wet bath 13 is composed preferably of quartz.

[0068] As illustrated in FIG. 4, the megasonic-wave oscillation plate 17is soaked in the wafer 14 filling the outer wet bath 15 therewith. Theouter wet bath 15 is formed centrally at a bottom thereof with anopening through which the megasonic-wave oscillator 18 is fit forslidable movement. A waterproof seal 20 is sandwiched between themegasonic-wave oscillator 18 and the opening for preventing leakage ofthe water 14.

[0069] The motor 19 is mechanically connected to the megasonic-waveoscillator 18 through a converter 19 a which converts rotational motioninto linear motion, such as rack and pinion. Accordingly rotationalmotion output from the motor 19 is converted to linear motion by theconverter 19 a, and then, the linear motion is transferred to themegasonic-wave oscillator 18. Hence, if the motor 19 rotates in a firstdirection (for instance, a clockwise direction), the megasonic-waveoscillation plate 17 and the megasonic-wave oscillator 18 move upwardly,whereas if the motor 19 rotates in a second direction (for instance, acounterclockwise direction), the megasonic-wave oscillation plate 17 andthe megasonic-wave oscillator 18 move downwardly.

[0070] An operation of the semiconductor cleaner 10 in accordance withthe first embodiment is explained hereinbelow.

[0071] The motor 19 moves the megasonic-wave oscillation plate 17 andthe megasonic-wave oscillator 18 upwardly or downwardly below a bottomof the inner wet bath 13 with the wafer 12 being kept soaked in thecleaning solution 11. While moving together with the megasonic-waveoscillation plate 17, the megasonic-wave oscillator 18 oscillates themegasonic-wave oscillation plate 17 to thereby emit megasonic waves fromthe megasonic-wave oscillation plate 17. The thus emitted megasonicwaves are irradiated to the wafer 12, and resultingly, the wafer 12 iscleaned.

[0072]FIG. 5 illustrates that megasonic waves 21 emitted from themegasonic-wave oscillation plate 17 are irradiated to the wafer 12. InFIG. 5, only the wafer 12, the inner wet bath 13, the megasonic-waveoscillation plate 17 and the megasonic-wave oscillator 18 areillustrated, and other parts are omitted for simplification of FIG. 5.

[0073] The megasonic waves 21 emitted from the megasonic-waveoscillation plate 17 include first megasonic waves 21 a emitted upwardlyfrom the megasonic-wave oscillation plate 17, and second megasonic waves21 b emitted obliquely upwardly from the megasonic-wave oscillationplate 17.

[0074] A part of the second megasonic waves 21 b is reflected at asidewall 13 a of the inner wet bath 13, and is directed towards insideof the inner wet bath 13 as reflected megasonic waves 21 c. As a result,the first megasonic waves 56 a and the reflected megasonic waves 56 cinterfere with each other at locations indicated with solid circles 57.This means that the silicon wafer 55 locally receives the megasonicwaves 56 having high intensity.

[0075] In the conventional semiconductor cleaner 50 illustrated in FIG.1, the first megasonic waves 56 a irradiated upwardly from theoscillation plate 53 and the reflected megasonic waves 56 c interferewith each other at the solid circles 57, and resultingly, the siliconwafer 55 is damaged at the solid circles 57.

[0076] This is because the oscillation plate 53 and the cleaning bath 52are kept stationary relative to each other, and hence, an intensity ofmegasonic waves is increased at uniform locations due to interference ofmegasonic waves with each other.

[0077] In contrast, the megasonic-wave oscillation plate 17 from whichmegasonic waves are irradiated is always made to move upwardly ordownwardly in the semiconductor cleaner 10 in accordance with the firstembodiment. Accordingly, since a relative distance between themegasonic-wave oscillation plate 17 and the inner wet bath 13 (exactly,between the megasonic-wave oscillation plate 17 and the wafer 12 keptsoaked in the inner wet bath 13) is varied, locations at which anintensity of megasonic waves is increased due to interference ofmegasonic waves with each other are not uniform unlike the conventionalsemiconductor cleaner 50. Hence, it is possible to prevent megasonicwaves from locally concentrating to the wafer 12, and thus, it ispossible to protect the wafer 12 from being damaged due to interferenceof megasonic waves.

[0078] The reason is explained in detail hereinbelow.

[0079] In FIG. 5, phases of first megasonic waves 22 a irradiated fromthe megasonic-wave oscillation plate 17 when the megasonic-waveoscillation plate 17 reaches its uppermost point are illustrated inbroken lines, and phases of second megasonic waves 22 b irradiated fromthe megasonic-wave oscillation plate 17 when the megasonic-waveoscillation plate 17 reaches its lowermost point are illustrated insolid lines.

[0080] In FIG. 5, locations at which the reflected megasonic waves 21 aand the first megasonic waves 22 a interfere with each other areindicated with hollow circles (◯), and locations at which the reflectedmegasonic waves 21 a and the second megasonic waves 22 b interfere witheach other are indicated with star marks (★). It is understood thatlocations at which megasonic waves interfere with each other arescattered relative to the solid circles 57 illustrated in FIG. 1.

[0081] As mentioned above, the semiconductor cleaner 10 in accordancewith the first embodiment makes it possible to scatter locations atwhich megasonic waves interfere with each other, and hence, reduce anintensity of megasonic waves locally concentrating to the wafer 12 dueto interference of megasonic waves with each other. As a result, it ispossible to prevent the wafer 12 from being damaged due to interferenceof megasonic waves with each other.

[0082] A distance by which the megasonic-wave oscillation plate 17 andthe megasonic-wave oscillator 18 move upwardly and downwardly isdependent on a size of the wafer and/or a size of the inner wet bath 13.According to the experiments having been conducted by the inventor, theabove-mentioned advantageous effects can be ensured regardless of sizesof the wafer 12 and the inner wet bath 13, if the distance is set equalto or greater than 2 mm, and equal to or smaller than 20 mm.

[0083] The megasonic waves 21 irradiated from the megasonic-waveoscillation plate 17 may have any frequency. However, according to theexperiments having been conducted by the inventor, the megasonic waves21 having a frequency in the range of 750 KHz to 1 MHz both inclusivecould clean the wafer 12 to the best degree. Accordingly, it would bepreferable that the megasonic-wave oscillator 18 oscillates themegasonic-wave oscillation plate 17 such that the megasonic waves 21having a frequency in the range of 750 KHz to 1 MHz both inclusive areirradiated from the megasonic-wave oscillation plate 17.

[0084] [Second Embodiment]

[0085]FIG. 6 is a top view of a semiconductor cleaner 30 in accordancewith the second embodiment, when viewed from upward.

[0086] In the semiconductor cleaner 30 in accordance with the secondembodiment, eight wafers (not illustrated) are arranged in the inner wetbath 13 in parallel with one another. The semiconductor cleaner 30 isdesigned to include first to eighth megasonic-wave oscillation plates 17a to 17 h and first. to eighth megasonic-wave oscillators 18 a to 18 hin association with eight wafers.

[0087] Each of the first to eighth megasonic-wave oscillation plates 17a to 17 h is identical in structure with the megasonic-wave oscillationplate 17 in the first embodiment, and each of the first to eighthmegasonic-wave oscillators 18 a to 18 h is identical in structure withthe megasonic-wave oscillator 18 in the first embodiment.

[0088] Accordingly, a distance by which each of the eight wafers movesupwardly and downwardly can be controlled for each of the eight wafers.

[0089] A single megasonic-wave oscillation plate may be used in place ofthe first to eighth megasonic-wave oscillation plates 17 a to 17 h.However, by using the first to eighth megasonic-wave oscillation plates17 a to 17 h in place of a single megasonic-wave oscillation plate, itwould be easier to control each of the megasonic-wave oscillationplates.

[0090] [Third Embodiment]

[0091]FIG. 7 illustrates a semiconductor cleaner 40 in accordance withthe third embodiment.

[0092] The semiconductor cleaner 40 in accordance with the thirdembodiment is comprised of an inner wet bath 43 which is filled withcleaning solution 41 and in which a wafer 42 to be cleaned is soaked, anouter wet bath 45 which is filled with water 44 and which surrounds theinner wet bath 43, and a megasonic-wave irradiator 46 arranged below abottom of the inner wet bath 43 for irradiating megasonic waves.

[0093] The megasonic-wave irradiator 46 is comprised of a megasonic-waveoscillation plate 47 located in the outer wet bath 45 and below a bottomof the inner wet bath 43 in parallel with a bottom of the inner wet bath43, and a megasonic-wave oscillator 48 connected to the megasonic-waveoscillation plate 47 for oscillating the megasonic-wave oscillationplate 47 to produce megasonic waves.

[0094] The wafer 42 is kept soaked in the cleaning solution 41 by meansof an appropriate holder (not illustrated).

[0095] The inner wet bath 43 is formed at an inner surface 43 a of asidewall thereof with irregularity having a surface roughness greaterthan a wavelength of megasonic waves irradiated from the megasonic-waveoscillation plate 47.

[0096] In general, if a wave had a wavelength smaller than a surfaceroughness of a wall at which the wave is reflected, the wave would beirregularly reflected. In the semiconductor cleaner 40 in accordancewith the third embodiment, megasonic waves irradiated obliquely upwardlyfrom the megasonic-wave oscillation plate 47 are reflected at the innersurface 43 a of the sidewall of the inner wet bath 43, and then,interfere with megasonic waves irradiated upwardly from themegasonic-wave oscillation plate 47. Since the megasonic wavesirradiated obliquely upwardly from the megasonic-wave oscillation plate47 are irregularly reflected at the inner surface 43 a, locations atwhich those megasonic waves interfere with each other are in a broaderarea than the conventional semiconductor cleaner 50 illustrated in FIG.1.

[0097] Hence, it is possible to reduce an intensity of the megasonicwaves from locally concentrating to the wafer 42 due to interference ofmegasonic waves with each other, and thus, it is possible to protect thewafer 42 from being damaged.

[0098] It should be noted that the inner wet bath 13 in thesemiconductor cleaner 10 in accordance with the first embodiment may bedesigned to have slight irregularity at an inner surface of a sidewallthereof.

[0099] Hereinbelow is explained an example of the semiconductor cleaners10 and 40 in accordance with the first and third embodiments.

[0100] In theory, if the cleaning solution 11 and 41 is comprised ofwater, supersonic waves such as megasonic waves are reflected at asurface of the water 11 or 41 by 99.9%, and the thus reflectedsupersonic waves are directed to inside of the inner wet bath 13 or 43.The supersonic waves directed to inside of the inner wet bath 13 or 43interferes with supersonic waves directed to a surface of the water 11or 41, resulting in that sound pressure is locally strengthened everypitch of a standing wave. This is explained in detail hereinbelow.

[0101] It is assumed that megasonic waves irradiated from themegasonic-wave oscillation plate 17 or 47 have a frequency of 950 KHz.

[0102] A phase velocity of supersonic waves in pure water is equal to1483 m/s at 20 degrees centigrade. A wavelength λ is calculated asfollows. $\begin{matrix}{{{Wavelength}\quad \lambda} = {{Phase}\quad {velocity}\quad {{C\quad\left\lbrack {m/s} \right\rbrack}/{Frequency}}\quad {F\quad\lbrack{Hz}\rbrack}}} \\{= {{1483/950} = {1.56\quad {mm}}}}\end{matrix}$

[0103] Accordingly, a pitch of a standing wave is calculated as follows.

1.56/2=0.78 mm

[0104] This is small enough to be canceled by a wave generated at asurface of the water 11 or 41 by supersonic waves. Accordingly, it ispossible to ignore non-uniformity in cleaning the wafer 12 or 42, causedby a pitch of a standing wave, that is, damages of the wafer 12 or 42caused by megasonic waves locally having a high intensity due tointerference of megasonic waves with each other.

[0105] Accordingly, reflected megasonic waves which may interferencewith non-reflected megasonic waves are limited to megasonic wavesreflected from either the inner wet bath 13, 43 or the wafer 12, 42.

[0106] In order to prevent cancellation of cleaning effects of megasonicwaves and/or interference of megasonic waves with each other while themegasonic-wave oscillation plate 17 moves upwardly or downwardly, itwould be necessary for the megasonic-wave oscillation plate 17 to moveat a rate which is not equal to multiples of a phase velocity ofmegasonic waves and multiples of a half of a phase velocity of megasonicwaves, and by a distance which is not equal to multiples of a wavelengthof megasonic waves and multiples of a half of a wavelength of megasonicwaves.

[0107] Herein, there is considered a specific example having thefollowing specification.

[0108] Frequency of megasonic waves: 950 KHz

[0109] Cleaning solution: Water

[0110] Temperature of cleaning solution: 20 degrees centigrade

[0111] Revolution speed of the motor 19: 10,000 per second

[0112] Gear ratio of the motor 19: 1:4

[0113] Material of which the inner wet bath 13 and 43 is composed:Quartz

[0114] In order to avoid a rate Ca of the megasonic-wave oscillationplate 17 from being equal to multiples of a phase velocity of megasonicwaves and multiples of a half of a phase velocity of megasonic waves, itis assumed that the rate Ca is equal to one-third ({fraction (1/3)}) ofa phase velocity C.

Ca=C/3=1483/3=494.33 [m/s]=4943.3 [cm/s]

[0115] A distance L which ensures the rate Ca of 4943.3 [cm/s] under theabove-mentioned conditions is calculated as follows.

L=Ca[cm/s]/the number of reciprocation=4943.3/2500=1.977 cm

[0116] Herein, the number of reciprocation is calculated as 2500,because the motor 19 rotates at a revolution speed of 10000/s and has agear ratio of 1/4.

[0117] Accordingly, a distance La by which the megasonic-waveoscillation plate 17 moves upwardly or downwardly is calculated as ahalf of the distance L.

La=L/2=1.977/2=0.989 [cm]

[0118] Since the distance La of 0.989 cm is not equal to multiples of awavelength of megasonic waves and multiples of a half of a wavelength ofmegasonic waves, the megasonic waves do not interfere with each other.

[0119] When the inner wet bath 13 or 43 is composed of quartz, the innerwet bath 13 or 43 is frequently dealt with hydrofluoric acid at an innersurface thereof in order to reduce a surface roughness thereof. Afterbeing dealt with hydrofluoric acid, the inner wet bath 13 or 43 usuallyhas a surface roughness of about 0.5 mm, which is smaller than awavelength of megasonic waves. Accordingly, megasonic waves aremirror-reflected at an inner surface of the inner wet bath 13 or 43, orpass through the inner wet bath 13 or 43.

[0120] Since the megasonic-wave oscillation plate 17 or 47 is arrangedbelow a bottom of the inner wet bath 13 or 43, it is necessary for abottom of the inner wet bath 13 or 43 to have such a surface roughnessas mentioned above.

[0121] As having been explained in the third embodiment, it is necessaryfor the inner surface 43 a of a sidewall of the inner wet bath 13 or 43to have a structure for preventing megasonic waves frommirror-reflecting in order to avoid interference of megasonic waves witheach other. As mentioned earlier, if a wave had a wavelength smallerthan a surface roughness of a wall at which the wave is reflected, thewave would be irregularly reflected. In order to utilize thisphenomenon, the inner wet bath 13 or 43 is designed to have a sidewallhaving the inner surface 43 a having a surface roughness of 3 mm. Sincethe inner wet bath 13 or 43 is composed of quartz, it is sufficientlypossible to design the inner wet bath 13 or 43 to have a sidewall havingthe inner surface 43 a having a surface roughness of 3 mm.

[0122] In summary, the example of the semiconductor cleaners 10 and 40in accordance with the first and third embodiments has the followingdimensions.

[0123] Frequency of megasonic waves: 950 KHz

[0124] Cleaning solution: Water

[0125] Temperature of cleaning solution: 20 degrees centigrade

[0126] Revolution speed of the motor 19: 10,000 per second

[0127] Gear ratio of the motor 19: 1:4

[0128] Material of which the inner wet bath 13 and 43 is composed:Quartz

[0129] Portion of the inner wet bath 13 and 43 to be dealt withhydrofluoric acid: only bottom

[0130] Surface processing of the inner wet bath 13 and 43: only sidewall(surface roughness of 3 mm)

[0131] Distance by which the megasonic-wave oscillation plate 17 or 47moves: 0.989 cm

[0132] While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by way of the present invention is not to be limitedto those specific embodiments. On the contrary, it is intended for thesubject matter of the invention to include all alternatives,modifications and equivalents as can be included within the spirit andscope of the following claims.

[0133] The entire disclosure of Japanese Patent Application No.2001-394489 filed on Dec. 26, 2001 including specification, claims,drawings and summary is incorporated herein by reference in itsentirety.

What is claimed is:
 1. A semiconductor cleaner comprising: (a) a wetbath which is filled with liquid and in which a wafer to be cleaned issoaked; (b) a megasonic-wave irradiator located outside said wet bathfor irradiating megasonic waves to said wafer to clean said wafer; and(c) a unit which moves said megasonic wave irradiator towards or awayfrom said wet bath.
 2. The semiconductor cleaner as set forth in claim1, wherein said unit moves said megasonic-wave irradiator by a distancein the range of 2 mm to 20 mm both inclusive.
 3. The semiconductorcleaner as set forth in claim 1, wherein said megasonic-wave irradiatorirradiates megasonic waves having a frequency in the range of 750 KHz to1 MHz both inclusive.
 4. The semiconductor cleaner as set forth in claim1, wherein said wet bath is formed at an inner surface thereof withirregularity having a surface roughness greater than a wavelength ofsaid megasonic waves.
 5. The semiconductor cleaner as set forth in claim1, wherein said unit moves said megasonic-wave irradiator at a ratewhich is not equal to multiples of a phase velocity of said megasonicwaves and multiples of a half of a phase velocity of said megasonicwaves, and said unit moves said megasonic-wave irradiator by a distancewhich is not equal to multiples of a wavelength of said megasonic wavesand multiples of a half of a wavelength of said megasonic waves.
 6. Asemiconductor cleaner comprising: (a) a wet bath which is filled withliquid and in which first to N-th wafers to be cleaned are soakedwherein N is an integer equal to or greater than two; (b) first to N-thmegasonic-wave irradiators located outside said wet bath and associatedwith said first to N-th wafers, respectively, for irradiating megasonicwaves to said first to N-th wafers to clean said first to N-th wafers;and (c) first to N-th units which move said first to N-th megasonic-waveirradiators towards or away from said wet bath, respectively.
 7. Thesemiconductor cleaner as set forth in claim 6, wherein said first toN-th units move said first to N-th megasonic-wave irradiators,respectively, by a distance in the range of 2 mm to 20 mm bothinclusive.
 8. The semiconductor cleaner as set forth in claim 6, whereinsaid first to N-th megasonic-wave irradiators irradiate megasonic waveshaving a frequency in the range of 750 KHz to 1 MHz both inclusive. 9.The semiconductor cleaner as set forth in claim 6, wherein said wet bathis formed at an inner surface thereof with irregularity having a surfaceroughness greater than a wavelength of said megasonic waves.
 10. Thesemiconductor cleaner as set forth in claim 6, wherein each of saidfirst to N-th units moves said megasonic-wave irradiator at a rate whichis not equal to multiples of a phase velocity of said megasonic wavesand multiples of a half of a phase velocity of said megasonic waves, andeach of said first to N-th units moves said megasonic-wave irradiator bya distance which is not equal to multiples of a wavelength of saidmegasonic waves and multiples of a half of a wavelength of saidmegasonic waves.
 11. A semiconductor cleaner comprising: (a) a wet bathwhich is filled with liquid and in which a wafer to be cleaned issoaked; and (b) a megasonic-wave irradiator located outside said wetbath for irradiating megasonic waves to said wafer to clean said wafer,said wet bath being formed at an inner surface thereof with irregularityhaving a surface roughness greater than a wavelength of said megasonicwaves.
 12. A semiconductor cleaner comprising: (a) an inner wet bathwhich is filled with liquid and in which a wafer to be cleaned issoaked; (b) an outer wet bath which is filled with liquid and in whichsaid inner wet bath is soaked; (c) a megasonic-wave irradiator locatedoutside said inner wet bath for irradiating megasonic waves to saidwafer to clean said wafer; and (d) a unit which moves saidmegasonic-wave irradiator towards or away from said inner wet bath. 13.The semiconductor cleaner as set forth in claim 12, wherein saidmegasonic-wave irradiator is comprised of: (c1) a megasonic-waveoscillation plate located in said outer wet bath and below a bottom ofsaid inner wet bath in parallel with said bottom of said inner wet bath;and (c2) a megasonic-wave oscillator connected at one end to saidmegasonic-wave oscillation plate and at the other end to said unit, saidmegasonic-wave oscillator being fit into an opening formed at a bottomof said outer wet bath for slidable movement.
 14. The semiconductorcleaner as set forth in claim 12, wherein said inner wet bath iscomposed of quartz.
 15. The semiconductor cleaner as set forth in claim12, wherein said unit moves said megasonic-wave irradiator by a distancein the range of 2 mm to 20 mm both inclusive.
 16. The semiconductorcleaner as set forth in claim 12, wherein said megasonic-wave irradiatorirradiates megasonic waves having a frequency in the range of 750 KHz to1 MHz both inclusive.
 17. The semiconductor cleaner as set forth inclaim 12, wherein said inner wet bath is formed at an inner surfacethereof with irregularity having a surface roughness greater than awavelength of said megasonic waves.
 18. The semiconductor cleaner as setforth in claim 12, wherein said unit moves said megasonic-waveirradiator at a rate which is not equal to multiples of a phase velocityof said megasonic waves and multiples of a half of a phase velocity ofsaid megasonic waves, and said unit moves said megasonic-wave irradiatorby a distance which is not equal to multiples of a wavelength of saidmegasonic waves and multiples of a half of a wavelength of saidmegasonic waves.
 19. A semiconductor cleaner comprising: (a) an innerwet bath which is filled with liquid and in which first to N-th wafersto be cleaned are soaked wherein N is an integer equal to or greaterthan two; (b) an outer wet bath which is filled with liquid and in whichsaid inner wet bath is soaked; (c) first to N-th megasonic-waveirradiators located outside said inner wet bath and associated with saidfirst to N-th wafers, respectively, for irradiating megasonic waves tosaid first to N-th wafers to clean said first to N-th wafers; and (d)first to N-th units which move said first to N-th megasonic-waveirradiators towards or away from said inner wet bath, respectively. 20.The semiconductor cleaner as set forth in claim 19, wherein each of saidfirst to N-th megasonic-wave irradiators is comprised of: (c1) amegasonic-wave oscillation plate located in said outer wet bath andbelow a bottom of said inner wet bath in parallel with said bottom ofsaid inner wet bath; and (c2) a megasonic-wave oscillator connected atone end to said megasonic-wave oscillation plate and at the other end toeach of said first to N-th units, said megasonic-wave oscillator beingfit into an opening formed at a bottom of said outer wet bath forslidable movement.
 21. The semiconductor cleaner as set forth in claim19, wherein said inner wet bath is composed of quartz.
 22. Thesemiconductor cleaner as set forth in claim 19, wherein said first toN-th units move said first to N-th megasonic-wave irradiators,respectively, by a distance in the range of 2 mm to 20 mm bothinclusive.
 23. The semiconductor cleaner as set forth in claim 19,wherein said first to N-th megasonic-wave irradiators irradiatemegasonic waves having a frequency in the range of 750 KHz to 1 MHz bothinclusive.
 24. The semiconductor cleaner as set forth in claim 19,wherein said wet bath is formed at an inner surface thereof withirregularity having a surface roughness greater than a wavelength ofsaid megasonic waves.
 25. The semiconductor cleaner as set forth inclaim 19, wherein each of said first to N-th units moves saidmegasonic-wave irradiator at a rate which is not equal to multiples of aphase velocity of said megasonic waves and multiples of a half of aphase velocity of said megasonic waves, and each of said first to N-thunits moves said megasonic-wave irradiator by a distance which is notequal to multiples of a wavelength of said megasonic waves and multiplesof a half of a wavelength of said megasonic waves.
 26. A semiconductorcleaner comprising: (a) an inner wet bath which is filled with liquidand in which a wafer to be cleaned is soaked; (b) an outer wet bathwhich is filled with liquid and in which said inner wet bath is soaked;and (c) a megasonic-wave irradiator located outside said inner wet bathfor irradiating megasonic waves to said wafer to clean said wafer; saidinner wet bath being formed at an inner surface thereof withirregularity having a surface roughness greater than a wavelength ofsaid megasonic waves.
 27. A method of cleaning a wafer soaked in liquidfilled in a wet bath, comprising the step of irradiating megasonic wavesto said wafer with a relative distance between said wafer and amegasonic-wave irradiator being varied.
 28. The method as set forth inclaim 27, wherein said relative distance is varied by reciprocating saidmegasonic-wave irradiator relative to said wafer.
 29. The method as setforth in claim 27, wherein said relative distance is varied in the rangeof 2 mm to 20 mm both inclusive.
 30. The method as set forth in claim27, wherein said megasonic waves have a frequency in the range of 750KHz to 1 MHz both inclusive.
 31. The method as set forth in claim 27,wherein relative distance is varied by moving said megasonic-waveirradiator at a rate which is not equal to multiples of a phase velocityof said megasonic waves and multiples of a half of a phase velocity ofsaid megasonic waves, and by a distance which is not equal to multiplesof a wavelength of said megasonic waves and multiples of a half of awavelength of said megasonic waves.